This invention relates to a vehicle wheel slip control method and, more particularly, to a method of controlling vehicle wheel slip when operating on a split coefficient of friction surface.
When a torque is applied to a vehicle wheel for braking or driving the wheel, a force is generated between the wheel and the road surface. This force is dependent upon various parameters, including the road surface conditions and the amount of slip between the wheel and the road surface. As the torque input to the wheel is increased, the force between the tire and the road surface increases as wheel slip increases, until a critical slip value is surpassed. When the slip exceeds this critical slip value, the force between the wheel and the road surface decreases and the wheel slip rapidly increases. Vehicle anti-lock braking systems and traction control systems attempt to control the wheel slip at a value around the critical slip value to maximize vehicle braking or acceleration.
When a vehicle is being braked or accelerated on a split coefficient of friction surface and an anti-lock braking system or traction control system is operating to individually maximize the force between each wheel and the road surface, the braking or driving torque between the road surface and the wheel on the higher coefficient side of the vehicle may be substantially greater than the torque between the road surface and the other wheel. This torque imbalance results in a yaw moment acting on the vehicle giving rise to a yaw rate.